The present invention relates to coated cemented carbide cutting tool inserts with a binder phase enriched surface zone, particularly useful for turning and drilling in steels and stainless steels.
Coated cemented carbide inserts with binder phase enriched surface zones are today used to a great extent for machining of steel and stainless materials. Through the binder phase enriched surface zone an extension of the application area is obtained.
Methods of producing binder phase enriched surface zones on cemented carbides containing WC, gamma phase. (Ti,Ta,Nb)C, and binder phase are known as gradient sintering and have been known for some time, e.g., through Tobioka (U.S. Pat. No. 4,277,283, Nemeth (U.S. Pat. No. 4,610,931), Taniguchi (U.S. Pat. No. 4,830,930), Okada (U.S. Pat. No. 5,106,674 and Gustafson (U.S. Pat. No. 5,649,279).
Conventional cemented carbide inserts are produced by powder metallurgical methods including milling of a powder mixture forming the hard constituents and the binder phase, pressing and sintering. The milling operation is an intensive milling in mills of different sizes and with the aid of milling bodies. The milling time is of the order of several hours up to several days. Such processing is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure. However, milling has its disadvantages. During the long milling time the milling bodies are worn and contaminate the milled mixture. Furthermore, even after an extended milling a random rather than an ideal homogeneous mixture may be obtained. Thus, the properties of the sintered cemented carbide containing two or more components depend on how the starting materials are mixed. Further, the extensive milling process generates a large fraction of very fine grained carbide particles that during the sintering process will cause a in many cases unwanted grain growth. The grain growth process often leads to the formation of a fraction of very large carbide particles especially of WC, which can deteriorate the thermomechanical properties of the cutting insert.
There exist alternative technologies to intensive milling for production of cemented carbide, for example, use of particles coated with binder phase metal. The coating methods include fluidized bed methods, solgel techniques, electrolytic coating, PVD coating or other methods such as disclosed in e.g. GB 346,473, U.S. Pat. Nos. 5,529,804 or 5,505,902. Coated carbide particles can be mixed with additional amounts of cobalt and other carbide powders to obtain the desired final material composition, pressed and sintered to a dense structure.
It has now surprisingly been found that cemented carbide inserts with binder enriched surface zone made from powder mixtures with cobalt coated hard constituents with narrow grain size distributions and without conventional milling have excellent cutting performance in steels and stainless steels in turning and drilling under both dry and wet conditions. Furthermore, it has been found that due to the very uniformly distributed binder phase on the carbide particles, it is possible to use a lower sintering temperature and still get a dense structure, especially valid at lower binder contents. It has also been found that a much higher cooling rate in combination with the lower sintering temperature gives the most optimal binder enriched surface structure for the application area mentioned above.
According to a first aspect, the present invention provides a coated cemented carbide comprising WC, 2-10 wt-% Co, 4-15 wt-% cubic carbides with a binder phase enriched surface zone essentially free of gamma phase, the WC forming grains, the WC-grains have an average grain size in the range 1.0-3.5 xcexcm, and that the number of WC-grains larger than 2 times the average grain size is less than 10 grains/cm2 measured on a representative polished section 0.5 cm2 in area, and the number of grains in area larger than 3 times the average grain size is less than 5 grains/cm2 as measured over the section area.